This invention relates to a support for a substrate disposed in a semiconductor processing chamber.
In many semiconductor device manufacturing processes, the required high levels of device performance, yield, and process repeatability can only be achieved if the substrate (e.g., a semiconductor wafer) remains relatively free of defects during processing of the substrate.
For example, consider rapid thermal processing (RTP), which is used for several different fabrication processes, including rapid thermal annealing (RTA), rapid thermal cleaning (RTC), rapid thermal chemical vapor deposition (RTCVD), rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN).
There is a trend in these processes to increase substrate size so as to increase the number of devices which can be fabricated simultaneously. If substrate thickness is constant, the mass of the substrate is proportional to the square of its radius or edge length.
In susceptor systems, the substrate is supported by being placed on a susceptor support. Thus, the amount of support is proportional to the surface area of the substrate. In susceptorless systems, the substrate is usually only supported around its perimeter with an edge ring.
We have discovered, however, that the edge ring has a tendency to impart a localized ring of scratches around the perimeter of the bottom surface of the substrate, which is explained as follows. A substrate tends to sag where it is not supported by the edge ring, i.e., in areas away from its edge, causing the substrate to assume a curved shape. Exposure of the substrate to high temperatures makes the substrate more susceptible to sag, thus increasing its curvature.
Because of its curvature, the edge of the substrate assumes a slight angle from the horizontal. For instance, the edge of a 12-inch (300 mm) diameter substrate may be about 150 xcexcm higher than its center at room temperature, thereby causing the edge of the substrate to assume an angle of about 0.1 degrees from the horizontal. Due to this angle, the substrate does not rest flat on the surface of the edge ring but instead contacts the ring""s inside edge. This condition is illustrated in FIG. 1 for a substrate 10 and an edge ring 20. As a result, edge ring tends to scratch the bottom surface of the substrate.
Larger, e.g., 12-inch (300 mm) substrates are especially susceptible to scratches for two reasons. First, larger substrates are heavier and more highly curved when supported at their edge, causing the substrate to contact the edge ring with greater force. Second, larger substrates are typically provided with backside surfaces that are highly polished, which tend to show scratches more readily than unpolished surfaces.
A ridge, or xe2x80x9cedge crownxe2x80x9d, present on the inside edge of the edge ring was also found to scratch the substrate. The edge crown, which is formed when the edge ring is coated with a layer of polysilicon, is composed of excess polysilicon that preferentially deposits on the inside edge of the edge ring. The edge ring is coated with a layer of polysilicon to render it opaque in the frequency range used for temperature measurements of the substrate, thereby improving the accuracy of the temperature measurement.
Scratches on the surface of a substrate are undesirable because they increase the susceptibility of the substrate to slip. Slip is a defect in the crystalline structure of the substrate which destroys any devices through which it may pass, thereby reducing the yield of the substrate. More particularly, the presence of scratches on a substrate causes slip to occur in the substrate at a lower temperature than if no scratches were present. In other words, the presence of scratches makes a substrate less robust and less able to tolerate high temperatures. Scratches also increase the susceptibility of a substrate to slip under rapidly varying temperature conditions. Scratches are therefore a particularly significant problem for substrates processed in RTP chambers, where temperatures typically exceed 1100xc2x0 C. and are subject to rapid change.
In addition to increasing the susceptibility of a substrate to slip, scratches also introduce undesirable cosmetic imperfections in the substrate. Furthermore, scratches may generate stray particles which may contaminate a device fabrication process, thereby decreasing yield.
Therefore, an object of the present invention is to provide a support for a substrate that minimizes the effect of scratches on the yield of a substrate.
Another object of the present invention is to provide a support for a substrate that minimizes its tendency to scratch the substrate.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.
The present invention is directed to a substrate support. The substrate support comprises a shelf, which may be annular, having a surface sloped at a first angle from the horizontal. The first angle is selected such that the substrate support contacts the substrate substantially at an edge portion of the substrate. The first angle of the shelf is greater than the angle of the edge portion of the substrate. For instance, the first angle may range from 1-10 degrees. The substrate support is provided with a sloped surface to shift the point at which it contacts the substrate towards the edge of the substrate, thereby causing scratches to be formed only near the edge of the substrate where they do not affect yield.
The surface of the shelf, viewed in cross-section, may be either planar, concave or convex. Furthermore, the cross-sectional shape of the surface may be spherical, parabolic, hyperbolic, or some other empirically derived shape.
The shelf may include a beveled portion, which may be annular, disposed on its inner edge. The beveled portion has a surface sloped at a second angle that is greater than the first angle of the shelf. The beveled portion is provided to prevent the substrate from contacting the ridge formed on the inner edge of the edge ring, thereby preventing the ridge from scratching the substrate.
The present invention may include a plurality of materials. The substrate support may be composed of silicon carbide. The substrate support may further include at least one coating disposed on a surface of the support. In one embodiment, the coating is a layer of polysilicon. In another embodiment, the coating is a layer of silicon nitride. In yet another embodiment, the coating does not cover the annular shelf.
The present invention may also include a plurality of surface finishes. A surface of the annular shelf may be machined or polished. Alternatively, a surface of the coating may be machined or polished.
Advantages of the present invention include the following. First, the present invention shifts the region where scratches are formed substantially to the edge of the substrate. Since the edge of the substrate contains no devices, the scratches do not adversely affect yield. Second, the present invention reduces the tendency of the support to scratch the substrate surface, thereby increasing the temperature at which the substrate may be processed without the occurrence of slip. As a result, substrates of up to 12 inches in diameter or more may be processed at the high temperatures necessary for such processes as RTP with satisfactory yield.